1. [Field of the Invention]
The present invention relates to an ultrasonic vibration tool for bonding overlapped portions of a plurality of members to be bonded together made from metals with ultrasonic vibration.
2. [Description of the Prior Art]
FIG. 25 shows the support structure of a tool by a holder disclosed by Japanese Patent Publication No. 2911394. In FIG. 25, reference numeral 251 denotes an ultrasonic horn having a length equal to the wavelength of resonance frequency (length from the maximum vibration amplitude point f13 to the maximum vibration amplitude point f17), 252 bonding working portions projecting outward from the ultrasonic horn 251 at the central maximum vibration amplitude point f15 of the ultrasonic horn 251, 253 a round bar-like booster having a length equal to the half of the wavelength of resonance frequency (length from the maximum vibration amplitude point f11 to the maximum vibration amplitude point f13), 254 a cylindrical support portion projecting outward from the booster 253 at the central minimum vibration amplitude point f12 of the booster 253, 255 a round bar-like booster which is symmetrical to the booster 253 and has a length equal to the half of the wavelength of resonance frequency (length from the maximum vibration amplitude point f17 to the maximum vibration amplitude point f19), 256 a cylindrical support portion which is symmetrical to the support portion 254 and projects outward from the booster 255 at the central minimum vibration amplitude point f18 of the booster 255, 257 a transducer, 258 a holder, 259 and 260 cylindrical clamp portions which have a slit in part of the wall and project downward from both end portions of the holder 258, 261 a mounting table placed right below the bonding working portion 252, and 262 and 263 a plurality of members to be bonded together which are made from metals. The booster 253 and 255 are connected coaxial to both ends of the ultrasonic horn 251 by unshown headless screws, and the transducer 257 is connected coaxial to the end portion of the booster 253 by an unshown headless screw. While the support portion 254 is inserted in the inside of the clamp portion 259 and the support portion 256 is inserted in the inside of the clamp portion 260, the clamp portions 259 and 260 are fastened by unshown bolts to narrow the widths of the slits and hold the support portions 254 and 256 from the outside, respectively. The holder 258 is attached to a piston rod which is a vertically movable output portion of a pressure unit such as an air cylinder. The member 262 and 263 to be bonded together are mounted on the mounting table 261 in such a manner that they are placed one upon the other. In this state, the holder 258 is moved down by the air cylinder, the bonding working portion 252 and the mounting table 261 hold the members 262 and 263 to be bonded together by pressure, ultrasonic vibration is transmitted from the transducer 257 to the ultrasonic horn 251 through the booster 253, the bonding working portion 252 vibrates in a direction shown by an arrow X, this vibration is transmitted from the bonding working portion 252 to the members 262 and 263 to be bonded together, the contact portions of the members 262 and 263 to be bonded together vibrate in a horizontal direction while they are pressed by the bonding working portion 252 and the mounting table 261, frictional heat is generated between the contact portions, and the contact portions of the members 262 and 263 to be bonded together are thereby activated and bonded together.
However, the above support structure of the prior art tool is such that the boosters 253 and 255 having a length equal to the half of the wavelength of resonance frequency are connected to both ends of the ultrasonic horn 251 which has a length equal to the wavelength of resonance frequency and the cylindrical support portions 254 and 256 of the boosters 253 and 255 are held by the cylindrical clamp portions 259 and 260 of the holder 258, respectively. That is, since the cylindrical support portions 254 and 256 are held by the cylindrical clamp portions 259 and 260, respectively, the distance between the support portion 254 and the other support portion 256 must be at least the total of the wavelength and the half of the wavelength of resonance frequency in order to secure as wide a working space as possible for the members 262 and 263 to be bonded together. Therefore, when the bonding working portion 252 and the mounting table 261 hold the members 262 and 263 to be bonded together by pressure, there is a possibility that the ultrasonic horn 251 and the boosters 253 and 255 may bend upward in an arc form with the support portions 254 and 256 as joints. If they bend, the vibration state, that is, resonance state goes wrong, thereby causing a bonding failure. To solve this problem, the present applicant developed a plate-like ultrasonic horn 271 shown in FIG. 26 and used it in place of the above ultrasonic horn 251. However, when the members 262 and 263 to be bonded together are a semiconductor chip and a circuit board and the semiconductor chip is surface mounted on the circuit board by ultrasonic vibration bonding, the basic weight to be applied to each bump (electrode) of the semiconductor chip is limited. Since the number of bumps is small, it is difficult to control the bonding weight at the time of bonding when the bonding weight (weight required for bonding) which is obtained by multiplying the number of bumps of the semiconductor chip by the basic weight is lower than the total weight of the boosters 253 and 255 and the ultrasonic horn 271. On the other hand, when the number of bumps is large and the bonding weight is high, there may occur a bonding failure that the ultrasonic horn 251 and the boosters 253 and 255 bend and some of the bumps of the semiconductor chip are not bonded to the pads (electrodes) of the circuit board properly.